To improve passenger comfort in vehicles such as automobiles, efforts have hitherto been made to reduce the incursion of noise and vibrations into the passenger compartment by installing various anti-vibration materials at places where noise and vibrations are generated. For example, by using, for the engine that is the main source of noise and vibrations in an automobile, anti-vibration rubber in components such as torsional dampers, engine mounts and muffler hangers, vibrations while driving the engine are absorbed and both the incursion of noise and vibrations into the passenger compartment and noise dissemination into the surrounding environment are reduced.
The basic properties required of such anti-vibration rubbers are strength properties for supporting a massive body such as an engine, and an anti-vibration performance which absorbs and suppresses vibrations from the body. In addition, when used in a high-temperature environment such as an engine compartment, the anti-vibration rubber, in addition to having, of course, excellent strength properties, a low dynamic-to-static modulus ratio and an excellent anti-vibration performance, is also required to have an excellent heat resistance, ozone resistance and compression set. In recent years, particularly with the trend toward higher engine power and with tighter space constraints in the engine compartment owing to factors such as expansion of the passenger compartment space, temperatures within the engine compartment have been trending upward, creating a more acute need for heat-resistance in automotive anti-vibration rubber.
In addition to the above, because automobiles are used even in high-latitude regions, automotive anti-vibration rubbers are also required to have good low-temperature properties.
Research on compounding specific amounts of a rubber component, a crosslinking system and other additives for anti-vibration rubber in order to impart such collectively outstanding properties is actively underway, and numerous patent applications have already been filed. Of these many patent applications, some make deliberate use of bismaleimide compounds to improve the crosslinking system. For example, JP-A H03-258840 discloses a rubber compound of excellent heat resistance and a low dynamic-to-static modulus ratio that is obtained by compounding sulfur, bismaleimide and a specific carbon black with a rubber component.
In addition, JP-A 2005-194501 discloses a rubber compound having excellent heat resistance, a low dynamic-to-static modulus and excellent durability through the use of a bismaleimide compound and a thiazole-type vulcanization accelerator.
However, although the above rubber compositions have excellent heat resistances and low dynamic-to-static modulus ratios, they leave something to be desired in terms of durability, compression set, low-temperature properties and processability. Also, in order to maintain for an extended period of time the spring characteristics which are strongly desired in anti-vibration rubber, it is necessary to minimize changes in moduli and further enhance the heat resistance.
The inventor earlier disclosed a rubber composition endowed with a low dynamic-to-static modulus ratio and excellent failure characteristics, heat resistance and durability by including, as vulcanizing agents: sulfur, a specific sulfur compound, and a bismaleimide compound (JP-A 2010-254872). However, even in this art, there remains room for improvement in the heat resistance and other properties of the anti-vibration rubber.